1. Field of the Invention
The present invention relates to an over voltage arrester and more particularly to such an arrester formed with a gas-tight housing with two electrodes supported by opposite ends of a tubular insulator member.
2. The Prior Art
Over voltage arresters are employed to trigger a discharge path between two electrodes when a voltage applied between the electrodes increases beyond a predetermined level. In order to provide a specific triggering voltage at which such arresters become operative, the interior of such an arrester is typically filled with a specific gas mixture, at a relatively low pressure, and has one or more conductive triggering strips or lines supported on the interior of the tubular insulator member which supports the electrodes. The strip must be in electrical contact with one of the electrodes, and extends from that electrode for a short distance toward the other electrode, along the surface of the tubular insulator member. The triggering of the gas-discharge path, when the arrester becomes operative, is facilitated by means of the conductive triggering strip or line. The use of radioactive materials with such arresters is avoided as much as possible. The triggering strips are typically applied to the tubular insulator member in the same manner as with luminescent tubes.
Over voltage arresters are designed to trigger as rapidly as possible in response to a rapidly increasing over voltage condition. In the arresters which are currently available employ one or more triggering lines which are soldered to one of the electrodes by the use of a metallic solder.
It has not been possible to use such triggering lines with arresters having insulators formed of glass or glazed ceramic materials, because it has not been possible to establish electrical contact between the triggering lines and the electrodes. The resistance of the triggering lines must be above 5,000 ohms, for the arresters to function properly, but the processes for establishing the required electrical connection of the triggering lines to an electrode by hard soldering are disadvantageous either because they lower the resistance of the triggering lines or of the tubular insulating member or because they are unduly costly. When ordinary solder or silver solder is used while a partial vacuum or reduced pressure is maintained within the arrester, the vapour pressure of the solder is so great that part of the solder is vaporized and condenses on the interior of the insulator member. The insulation resistance of the ceramic insulator falls below 10.sup.8 ohms after such an assembly, with knob type arresters, which is too low for normal operation. This disadvantage can be prevented only by use of a very costly manufacturing process employing a solder with a very low vapour pressure such as pure gold, or else the arrester is first soldered while the interior thereof is filled with pressurized hydrogen gas, to suppress vaporization of the solder, after which the excess hydrogen gas is removed in a high-vacuum at a low temperature, by diffusion through a platinum wall.
It is, therefore, desirable to provide a means for forming a suitable over voltage arrester employing a ceramic or glass insulator member which avoids the disadvantages of the prior art.